The goal of the proposed work is to develop a new generation of commercial ultrasound applicators and associated hardware/software capable of improved heating uniformity and controllability at various depths within the body, with specific application, for the first time, to thermal therapy for breast cancer and chest wall recurrence. Such thermal therapy, used in conjunction with conventional radiation therapy, has been shown to be safe and effective in a number of clinical studies. These new applicator designs are based on extensive clinical experience with the SONOTHERM(R) 1000 multichannel ultrasound hyperthermia system, an FDA approved system now in clinical use in over 35 clinical sites around the world. The proposed applicators will be-developed specifically for use with that system, though some hardware and software modifications will be necessary to accept the new applicators. The goals of research in Phase I are to improve the performance of the SONOTHERM (R) system for treatment of breast and chest wall lesions by examining: (a) higher frequency applicator operation to limit penetration depth where indicated; (b)simultaneous multifrequency operation to provide depth of penetration varying over the face of the ultrasound applicator array; and (c) improved means for coupling these new applicators for breast and chest wall treatments. Phase II will be devoted to the construction of clinical testing versions of applicators and other ancillary hardware and software developed during Phase I, and to conduct FDA sanctioned clinical trials at selected sites within the SONOTHERM(R) 1000 user base in the U.S. to determine the safety and effectiveness of the new devices.